1. Field of the Invention
This invention relates to helicopter rotor blades and is particularly concerned with rotor blades of the construction disclosed in GB-A-1538055.
2. Description of the Prior Art
Rotor blades according to GB-A-1538055 are characterised by a swept tip portion which in plan has a forwardly swept leading edge portion, a rearwardly swept leading edge portion, a rearwardly swept extreme tip edge and a rearwardly swept trailing edge. Such rotor blades have proved highly successful in producing large increases in the allowable rotor operating envelope, and were a significant factor in the establishment of the world absolute speed record for helicopters of 249.10 mph (400.81 km/hr) set by a Westland Lynx helicopter in 1986.
In continuing efforts to improve helicopter performance, wind tunnel tests of the prior rotor blades revealed an area of separated flow on an upper surface of the blade behind the notch region (i.e. the region where the forwardly swept leading edge portion of the tip joins the leading edge of the blade). The problem occurs at high angles of incidence in a range typically encountered by the retreating blade of a helicopter sustaining rotor, and the size of the separated area increases as incidence is increased.
Prior art means for maintaining attached flow include the vortex generating devices of U.S. Pat. No. 4,655,419 for fixed wing applications and U.S. Pat. No. 4,455,045 for fixed wing and road vehicle applications. In the former the vortex generators comprise upstanding generally planar members positioned near the maximum thickness of the aerofoil and in the latter comprise generally triangular shaped channels located at the trailing edge of a wing. Tests showed that such arrangements were unsuitable for preventing separated flow behind the notch region of a helicopter rotor blade of the construction disclosed in GB-A-1538055.
An objective of this invention is therefore to reduce or eliminate the area of separated flow behind the notch region of a helicopter rotor blade of the construction disclosed in GB-A-1538055.